1. Field of the Invention
This invention relates to an adhesion promoter for use in ultraviolet radiation curable siloxane compositions and the resulting compositions containing same.
2. Background Information
The use of certain silanes as coupling agents to enhance the adhesion of a variety of organic and silicone compositions to substrates is known in the art. While these known silanes and certain other silicon containing compounds do function as coupling agents and adhesion promoters, they do not function equally in all situations. Whether a particular silane is useful as an adhesion promoter can depend on the type of substrate, the type of composition or material being bonded to the substrate, and the conditions under which the adhesive bond is to perform. Included in these silane coupling agents are those which have organofunctional groups bonded to the silicon atom, such as a vinyl functional group, vinyl or methacrylate and also bonded to the silicon atom are hydrolyzable groups, such as methoxy or ethoxy.
The silane coupling agents have found use in certain ultraviolet radiation curable compositions, such as described by Blyler et al, in Chapter 10, of "Optical Fiber Telecommunications", by Miller et al, Academic Press, 1979, pages 314 to 316. Blyler et al state that the requirements for coupling agents are that they have two functionalities, one capable of interacting with the resin phase and one favoring reaction with the substrate. Blyler et al are specifically discussing coupling agents for use in UV curing epoxy acrylate and specifically suggest using a styrene-functional amine hydrochloride silane of the formula ##STR1## The functional organic moeity such as the vinyl group may form covalent bonds with the corresponding reactive groups in the resin phase, leading to a bond which should provide a high degree of water resistance.
Vazirani in U.S. Pat. No. 4,099,837, issued Jul. 11, 1978, teaches that coupling agents can be used in UV cured epoxy-acrylate coatings. Vazirani reports that the most effective coupling agents for fiber coating also have unsaturated sites that attach to the polymer and improve the bond between the polymer coating and the glass surface and that coupling agents proposed for optical fiber coatings are typically silanes and titanates. Vazirani specifically uses the styrene-function amine hydrochloride silane described by Blyler et al.
The need for an adhesion promoter useful in ultraviolet radiation curable compositions is present today. The inventors being faced with the problem of increasing the adhesion of UV curable siloxane compositions to a variety of substrates undertook an investigation which lead to the present inventive solution. The adhesion sought was of the unprimed nature, such that the UV curable composition applied to a substrate would develop the required adhesion without the use of a primer. The adhesion should develop faster after cure of the composition has taken place, instead of like some situations of the prior art in which substantial period of time are required before the desired adhesion is obtained. The adhesion developed should be strong enough that the cured composition exhibits cohesive failure. Other characteristics of the desired adhesion were that it should retain its strength under high moisture conditions and it should not be corrosive to electrical and electronic devices.
An area where adhesion is needed is the optical fiber communications industry. The optical fibers carry the information by light waves. These fibers are made of glass coated with a primary coating to preserve the strength and performance of the fiber by protecting it from adverse mechanical, thermal, and chemical environments. Current commercially available ultraviolet radiation curable coatings are deficient with respect to adhesion of the primary coating to the glass surface. This industry can use two kinds of adhesion, namely strong adhesion and controlled adhesion. In the case of controlled adhesion, the coating should adhere to the glass fiber, but not so strongly so that it will not come off mechanically when desired during subsequent operations. One major limitation of current commercially available organic optical fiber coatings is their poor adhesion in the presence of moisture. Ultraviolet radiation cured silicone optical fiber coatings have poor adhesion to glass. In the case of strong adhesion meaning a bond which exhibits cohesive failure, the interface between the glass and coating is strong enough that the coating is removable by chemical means during subsequent operations rather than by mechanical means. This type of adhesion should also remain in the presence of moisture.